I would like to relocate the flue for my gas water heater

I have a two story house in S Cal with a 3 car garage. The second floor extends over two of the garage stalls but the outer most stall has a small attic space above it. The gas water heater is located in the rear corner of this stall and when they built the house, it looks like they took the easy way out and ran the flue for the water heater fully exposed in the garage. The vent runs over and then up into the attic where it then enters an interior wall and goes up to the 2nd floor roof.

I would like to reroute the vent so it is mostly in the attic space above.

There appears to be plenty of room. The horizontal run is 6 1/2 feet, the top of the water heater to the drywall ceiling is 27" and in the attic, from the top of the ceiling to where the vent enters the wall is about 18". Also, there won't be a problem with the trusses in the attic.

Assuming there wouldn't be any problems in doing this, I would like to start by making the connection to the wall first and run over to directly above the water heater.

What should the drop/slope of the vent be over this 6 1/2 feet? Once I figure that out, I can drop directly down to the heater

Also, the diameter of the pipe when it leaves the water heater is 4" (13" circ) and then transitions to 5" (15" circ). Is there a reason for this?

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I would like to relocate the flue for my gas water heater

Sure, you can move it. Consider the following:

1. Single wall flue, which is what comes out of your water heater, cannot penetrate the ceiling. You MUST transition to a double wall type B flue at the ceiling and into the attic. Very important. Type B flues will be labeled as such.

2) Keep as much vertical run in the flue as possible. With a single vent, which this is (as opposed to a two appliance combination vent), you can run pretty far, as long as you're pitching upward. Just remember vertical is gooood. No flatter than 1/4" per foot, and that is pushing it if you ask me.

3) Maintain at least 1" clearance between the type B flue and any combustible surface, including sheetrock. Use sheet metal collars at penetrations to maintain clearance.

4) You must terminate the flue to the exterior, not the attic. Be sure!

5) In the attic, don't pack insulation around the pipe. The code requires an "insulation shield", which is usually a hoop of sheet metal that maintains a 1" airspace around the outer wall of the type B flue and prevents insulation from building up against the flue. The flues get hot, so this encourages air circulation.

6) Use heat (foil) tape on all your single wall flue joints to seal them nice and tight.

I would like to relocate the flue for my gas water heater

The picture on the right (in the garage) appears to be double wall type B flue. Where I'm getting lost is it seems to transition into a rectangular type of duct where it passes through the sheetrock. Now, this could be a regional thing that I just don't see here, but it looks like single wall rectangular DUCT (not flue) material, which should not ever be used to vent combustion gases. NEVER! In the picture on the left it appears to be rectangular, but I can't tell. If it fits in the wall, I'm very suspicious. A type B vent for a water heater is usually going to be about 6" outside diameter and round-shaped.

Where is the uppermost vent termination? Roof or sidewall of the attic? I assume it vents to the outdoors?

Your next step is to determine if my assumption is correct. If it is, you need to abandon the existing vent and re-route it using type B vent. It is not difficult...The stuff twist-locks together and can be secured with very short sheet metal screws at the joints. The screws are long enough to penetrate the outer wall, but they don't go into the inner wall. Reasonably easy DIY project if you're a little bit handy.

Really, the hardest part of doing this is penetrating the roof and making sure you do it correctly so you don't cause leaks. Depending on your roof's pitch, there are minimum height requirements for the vent termination. If it is too low, there is a good chance that it will not vent correctly. If you know your roof's pitch I can give you the minimum height requirement. If you don't know the pitch, post a pic and we can guess to get you close.

Not to scare you, but remember that this vent is carrying carbon monoxide. You've certainly heard of it, and are probably aware of the hazards. Improperly vented water heaters kill people, and they never see it coming. I get a lot of complaining because I require a permit and an inspection for water heater replacements in my jurisdiction, but people often don't realize the imminent danger involved in an incorrectly installed water heater or vent. What I'm saying is that if you aren't sure, take more pics and post them, or involve an HVAC contractor.

How old is the house? What state? Was it inspected during construction? Still under builder's warranty?

I would like to relocate the flue for my gas water heater

Yup, I think you might be right. That looks a lot like what is in your picture! Good news I'd say. Sorry to spook you, but that is something that I have never seen in THOUSANDS of houses in this area...Learn something every day! Looks like some of the major manufacturers make a similar shape as well.